Heat exchanger and method of making it

ABSTRACT

A heat exchanger is constructed by tubes, corrugated fins and head pipes, which are assembled together. Herein, the tube is constructed by bending a flat plate whose surfaces are clad with brazing material to form a first wall and a second wall, which are arranged opposite to each other with a prescribed interval of distance therebetween to provide a refrigerant passage. Before bending, a number of swelling portions are formed to swell from an interior surface of the flat plate by press. By bending, the swelling portions are correspondingly paired in elevation between the first and second walls, so their top portions are brought into contact with each other to form columns each having a prescribed sectional shape corresponding to an elliptical shape or an elongated circular shape each defined by a short length and a long length. The columns are arranged to align long lengths thereof in a length direction of the tube corresponding to a refrigerant flow direction such that obliquely adjacent columns, which are arranged adjacent to each other obliquely with respect to the length direction of the tube, are arranged at different locations and are partly overlapped with each other with long lengths thereof in view of a width direction perpendicular to the length direction of the tube. The tubes, corrugated fins and head pipes are assembled together and are then placed into a heating furnace to heat for a prescribed time.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to heat exchangers which are applicable toair conditioners particularly used for vehicles. In addition, thisinvention also relates to methods of manufacturing the heat exchangers.

[0003] This application is based on patent application Ser. No. Hei11-153022 filed in Japan, the content of which is incorporated herein byreference.

[0004] 2. Description of the Related Art

[0005] In general, heat-exchanger tubes are used for heat exchangerswhich are installed in air conditioners of vehicles, for example. Theheat-exchanger tubes are mainly classified into two types of tubes (orpipes), which are shown in FIGS. 19 and 20 respectively.

[0006]FIG. 19 shows an example of a so-called “seam welded tube”, whichis designated by a reference numeral “1”. That is, the seam welded tube1 is constructed by a tube 2 having a flat shape and a corrugated innerfin 4. Herein, the corrugated inner fin 4 is inserted into the tube 2 byway of its opening 3. The corrugated inner fin 4 is formed in acorrugated shape having waves whose crest portions “4 a” are bonded toan interior surface of the tube 2 by welding or else.

[0007]FIG. 20 shows an example of an extrusion tube, which is designatedby a reference numeral “5”. The extrusion tube 5 has tube portions “6”and partition walls “7”, which are integrally formed by extrusionmolding.

[0008] If a heat exchanger is designed using the seam welded tube 1shown in FIG. 19, it has an advantage in which since the corrugatedinner fin 4 is inserted into the tube 2, an overall heating area isenlarged to improve a heat transfer rate. However, there is adisadvantage in which manufacturing such a heat exchanger needs muchworking time in insertion of the corrugated inner fin 4 into the tube 2and welding of the corrugated inner fin 4 being bonded to the interiorsurface of the tube 2. This causes a problem in which manufacturing costis increased by human works.

[0009] If a heat exchanger is designed using the extrusion tube 5 shownin FIG. 20, it has an advantage in which since the partition walls 7 areformed to partition an inside space of the extrusion tube 5 intomultiple tube portions 6, an overall heating area is enlarged to improvea heat transfer rate. The extrusion tube 5 is manufactured using anextrusion molding technique. So, it is difficult to make the tubeportions 6 small so much, and it is difficult to make thickness of thepartition walls 7 sufficiently thin. In addition, the extrusion moldingtechnique needs an increasing amount of materials used for formation ofthe extrusion tube 5, so that manufacturing cost is being increased.Further, it is impossible to improve heat-exchange capability so muchdue to relatively large thickness of the partition walls 7.

SUMMARY OF THE INVENTION

[0010] It is an object of the invention to provide a heat exchanger thatis improved in pressure strength and heat-exchange capability withoutincreasing manufacturing cost so much.

[0011] It is another object of the invention to provide a method formanufacturing the heat exchanger.

[0012] A heat exchanger is constructed by tubes, corrugated fins andhead pipes, which are assembled together. Herein, the tube isconstructed by bending a flat plate whose surfaces are clad with brazingmaterial to form a first wall and a second wall, which are arrangedopposite to each other with a prescribed interval of distancetherebetween to provide a refrigerant passage. Before bending, a numberof swelling portions are formed to swell from an interior surface of theflat plate by press. By bending, the swelling portions arecorrespondingly paired in elevation between the first and second walls,so their top portions are brought into contact with each other to formcolumns each having a prescribed sectional shape corresponding to anelliptical shape or an elongated circular shape each defined by a shortlength and a long length. The columns are arranged to align long lengthsthereof in a length direction of the tube corresponding to a refrigerantflow direction such that obliquely adjacent columns, which are arrangedadjacent to each other obliquely with respect to the length direction ofthe tube, are arranged at different locations and are partly overlappedwith each other with long lengths thereof in view of a width directionperpendicular to the length direction of the tube. The tubes, corrugatedfins and head pipes are assembled together and are then placed into aheating furnace to heat for a prescribed time.

[0013] Because of the aforementioned arrangement and formation of thecolumns inside of the tube, it is possible to improve an overall heattransfer rate of the tube on the average, and it is possible to improvea pressure-proof strength with respect to the tube.

[0014] Incidentally, each of the columns has the prescribed sectionalshape which is defined by a relationship of$2.0 \leq \frac{2}{1} \leq {3.0.}$

[0015] In addition, using a first center distance p1 being measuredbetween the obliquely adjacent columns in the width direction of thetube and a second center distance p2 being measured between theobliquely adjacent columns in the length direction of the tube, thecolumns are arranged inside of the tube to meet relationships of$1.5 \leq \frac{p1}{d1} \leq {3.0\quad {and}\quad 0.5} \leq \frac{p2}{d2} \leq {1.5.}$

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] These and other objects, aspects and embodiments of the presentinvention will be described in more detail with reference to thefollowing drawing figures, of which:

[0017]FIG. 1 is a front view showing a heat exchanger in accordance witha first embodiment of the invention;

[0018]FIG. 2 is an enlarged perspective view showing a detailedconstruction of a tube which is an essential part of the heat exchangerof FIG. 1;

[0019]FIG. 3 is a sectional view of the tube taken along a line III-IIIin FIG. 2;

[0020]FIG. 4 is a sectional view of the tube take along a line IV-IV inFIG. 2;

[0021]FIG. 5 is a plan view partly in section showing an end portion ofthe tube being inserted into a head pipe;

[0022]FIG. 6A is a perspective view showing a flat plate;

[0023]FIG. 6B is a perspective view showing the flat plate subjected topress working;

[0024]FIG. 6C is a perspective view showing the flat plate being bent toconstruct a tube;

[0025]FIG. 6D is a perspective view showing that the tube and acorrugated fin are assembled together with a head pipe;

[0026]FIG. 7 is a graph showing comparison between column bodies havingelliptical and circular shapes in section, which are placed in a flowfield, with respect to a relationship between a surface flow length anda surface local heat transfer rate;

[0027]FIG. 8 is a graph showing comparison between the column bodieswith respect to a relationship between Reynolds number and dragcoefficient;

[0028]FIG. 9 is a graph showing comparison between a tube havingelliptical columns and an extrusion tube with respect to a relationshipbetween refrigerant circulation and heat transfer rate;

[0029]FIG. 10 is a graph showing comparison between the tube having theelliptical columns and extrusion tube with respect to a relationshipbetween refrigerant circulation and pressure loss;

[0030]FIG. 11A is a sectional view of a tube 11A containing columnstherein;

[0031]FIG. 11B is a sectional view of a tube 11B containing columnstherein;

[0032]FIG. 11C is a sectional view of a tube 11C containing columnstherein;

[0033]FIG. 11D is a sectional view of a tube 11D containing columnstherein;

[0034]FIG. 12 is a graph showing comparison between the tubes 11A, 11B,11C and 11D with respect to a relationship between refrigerantcirculation and heat transfer rate;

[0035]FIG. 13 is a graph showing comparison between the tubes 11A, 11B,11C and 11D with respect to a relationship between refrigerantcirculation and pressure loss;

[0036]FIG. 14 is a sectional view of a tube containing columns used in aheat exchanger in accordance with a second embodiment of the invention;

[0037]FIG. 15 is a sectional view of a tube containing columns andsemi-columns used in a heat exchanger in accordance with a thirdembodiment of the invention;

[0038]FIG. 16 is a plan view showing a modified example of the tube usedfor the heat exchanger of the third embodiment;

[0039]FIG. 17 is a sectional view of a tube containing columns havingdifferent shapes and sizes used in a heat exchanger in accordance with afourth embodiment of the invention;

[0040]FIG. 18 is a plan view of a refrigerant passage unit, which is anessential part of a heat exchanger of a fifth embodiment of theinvention;

[0041]FIG. 19 is a perspective view showing an example of a seam weldedtube which is conventionally used for a heat exchanger; and

[0042]FIG. 20 is a perspective view showing an example of an extrusiontube which is conventionally used for a heat exchanger.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] This invention will be described in further detail by way ofexamples with reference to the accompanying drawings.

[0044] [A] First Embodiment

[0045] Now, a heat exchanger will be described in accordance with afirst embodiment of the invention with reference to FIGS. 1 to 13.

[0046]FIG. 1 is a front view showing a heat exchanger 10, which isdesigned in accordance with the first embodiment of the invention.Herein, the heat exchanger 10 is constructed by tubes 11 each having aflat shape, a pair of head pipes 12, 13 and corrugated fins 14. The headpipes 12, 13 are arranged in contact with both ends of the tubes 11,wherein they communicate with refrigerant passages inside of the tubes11 respectively. Each of the corrugated fins 14 is arranged between thetubes 11, wherein crest portions are brought into contact with the tubes11.

[0047] An inside space of the head pipe 12 is partitioned into twosections (hereinafter, referred to as an upper section and a lowersection) by a partition plate 15, which is arranged slightly below acenter level of the head pipe 12. A refrigerant inlet pipe 16 isinstalled to communicate with the upper section of the head pipe 12,while a refrigerant outlet pipe 17 is installed to communicate with thelower section of the head pipe 12.

[0048] An overall front area of the heat exchanger 10 is divided intotwo areas (i.e., an upper area “a” and a lower area “b”) by thepartition plate 15. Refrigerant is introduced to flow in the tubes 11 indifferent directions (A) in connection with the two areas. With respectto the upper area “a”, refrigerant flow in a direction from the headpipe 12 to the head pipe 13. With respect to the lower area “b”,refrigerant flow in another direction from the head pipe 13 to the headpipe 12.

[0049] Each of the tubes 11 is constructed as shown in FIG. 2. That is,the tube 11 is made by bending a flat plate 20 to form a first wall 21and a second wall 22, which are arranged opposite to each other and inparallel with each other. So, a refrigerant passage 23 is formed in aspace being encompassed by the walls 21, 22.

[0050] A number of dimples 24 are formed on exterior surfaces of thetube 11 and are made by applying external pressures to the walls 21, 22to cave in at selected positions. Because of formation of the dimples24, a number of swelling portions 25 are correspondingly formed to swellfrom interior surfaces of the tube 11 within the refrigerant passage 23.

[0051] A top portion 25 a of the swelling portion 25 has an ellipticalshape in plan view being defined by a short length (or short diameter)and a long length (or a long diameter), which is placed along a lengthdirection (i.e., “A” in FIG. 2) of the tube 11. As for two swellingportions 25 which are arranged opposite to each other, their topportions 25 a are brought into contact with each other as shown in FIG.3. That is, the two swelling portions 25 whose top portions 25 a arebrought into contact with each other are connected together to form acolumn 26 which is provided between the first and second walls 21, 22and whose section has an elliptical shape. Incidentally, the sectionalshape of the column 26 is not necessarily limited to the ellipticalshape, so it can be formed like an elongated circular shape, forexample. In addition, the column 26 is not necessarily made in a hollowshape, so it is possible to make the column 26 solid.

[0052] The swelling portions 25 are arranged to adjoin with each otheras shown in FIG. 4. Herein, adjacent swelling portions, which arearranged adjacent to each other obliquely with respect to the directionA, are arranged in a zigzag manner while being partially overlapped witheach other in view of a direction perpendicular to the direction A.Therefore, the columns 26 are correspondingly arranged in a zigzagmanner in conformity with the swelling portions 25.

[0053] In FIG. 2, an air inlet direction by which air is introduced toperform heat exchange coincides with a width direction B of the tube 11.The tube 11 has a front-end portion 30 and a back-end portion 31, whichare arranged apart from each other in the air inlet direction. Inaddition, splitter plates 32, 33 are formed together with the front-endportion 30 and the back-end portion 31 respectively. Each of thesplitter plates 32, 33 is formed in prescribed thickness which isrelatively thin to act as a flow straightener for straightening an inletair flow around the tube 11.

[0054] As shown in FIG. 1, both ends of the tube 11 are inserted intothe head pipes 12, 13 respectively. Specifically, FIG. 5 shows that oneend of the tube 11 is inserted into the head pipe 13. To actualizeinsertion, cut sections 34, 35 are formed by partly cutting out thesplitter plates 32, 33 of the tube 11. That is, each end of the tube 11has a prescribed end shape, by which it is inserted into the head pipe(12 or 13).

[0055] A number of tube insertion holes 36 are formed at selectedpositions on surfaces of the head pipes 12, 13. Each tube insertion hole36 coincides with the end shape of the tube 11 to enable insertion ofthe tube 11 therein. To guide insertion of the tube 11, channels 37 (seeFIG. 6D) are formed at both ends of the tube insertion hole 36 to allowcut ends of the splitter plates 32, 33 of the tube 11 being insertedtherein.

[0056] The tube insertion hole 36 has an elongated shape whose width w1substantially coincides with width w2 of the end portion of the tube 11in which the cut sections 34, 35 are formed. In addition, an overallwidth w3 of the tube 11 including the splitter plates 32, 33 is madelarger than the width w1 of the tube insertion hole 36. Thus, when theend portion of the tube 11 is inserted into the tube insertion hole 36,the cut ends of the splitter plates 32, 33 of the tube 11 collide withthe head pipe (12 or 13) so that the tube 11 is prevented from beinginserted into the tube insertion hole 36 further more.

[0057] Next, a description will be given with respect to a method formanufacturing the heat exchanger 10 with reference to FIGS. 6A to 6D.

[0058] At first, a flat plate (or sheet metal) 20 shown in FIG. 6A isprepared for manufacture of the tube 11. Brazing material is clad on thesurfaces of the flat plate 20, which are made as an interior surface andan exterior surface of the tube 11 being manufactured. In addition,prescribed sections are cut from selected end portions of the flat plate20 in advance, wherein they are designated as the cut sections 34, 35.

[0059] Next, the flat plate 20 is subjected to press working or rollworking to form swelling portions 25 in connection with a refrigerantpassage 23 as shown in FIG. 6B. In addition, a bending overlap width 40is formed in connection with a front-end portion 30, while brazing tabs41 are formed in connection with a back-end portion 31. Then, the flatplate 20 is bent along with a center line of the bending overlap width40, which is shown in FIG. 6C. As the flat plate 20 is being bent, thebending overlap width 40 is folded so that two parts thereof come inconnection with each other, while the brazing portions 41 areapproaching each other and are then brought in contact with each other.Further, top portions 25 a of the swelling portions 25 are brought incontact with each other. Thus, it is possible to form the tube 11 havinga flat shape.

[0060] Next, there is prepared a head pipe 12 (or 13) having tubeinsertion holes 36 as shown in FIG. 6D. Herein, an end portion of thetube 11 is inserted into the tube insertion hole 36 of the head pipe 12(or 13). In addition, a corrugated fin 14 is arranged between adjacenttubes 11 in elevation, so that a heat exchanger 20 is being assembled.Thereafter, the assembled heat exchanger 10 is put into a heatingfurnace (not shown), wherein it is heated for a certain time with aprescribed temperature. So, the brazing material clad on the surfaces ofthe flat plate 20 (i.e., tube 11) is melted, so that parts of the heatexchanger 10 are subjected to brazing. That is, brazing is performed ontwo parts of the bending overlap width 40, the brazing portions 41 andthe top portions 25 a of the swelling portions 25, all of which arerespectively bonded together. In addition, brazing is performed betweenthe end portion of the tube 11 and the tube insertion hole 36, which arebonded together. Further, brazing is performed to actualize bondingbetween the tube 11 and crest portions of the corrugated fin 14, whichare brought in contact with each other when the corrugated fin 14 isarranged in connection with the tube 11.

[0061] In the heat exchanger 10 described above, each of columns 26which are arranged inside of the refrigerant passage 23 has a prescribedsectional shape corresponding to an elliptical shape whose long lengthmatches with the direction A. Thus, it is possible to improve a heattransfer rate while reducing flow resistance. Concretely speaking, arefrigerant flow may firstly collide with a front-end portion of thecolumn 26 in which curvature becomes small along side surfaces. Thus,refrigerant flow is accelerated in flow velocity to progress from thefront-end portion of the column 26 along its side surfaces. So, it ispossible to improve a local heat transfer rate. Then, the refrigerantflow passes by the front-end portion to reach a back-end portion of thecolumn 26. In that case, curvature becomes large along the side surfaceswith respect to the back-end portion of the column 26. This hardlycauses flow separation in which an eddy flow is separated from a mainflow in the refrigerant flow. That is, it is possible to suppress shaperesistance of the column 26 being small, so it is possible to reduceflow resistance.

[0062] Next, comparison is made between column bodies whose sectionalshapes correspond to a circular shape and an elliptical shaperespectively and which are arranged in flow fields. Herein, the columnbody having the elliptical shape in section is arranged in the flowfield in such a way that a long length matches with a flow direction. Inaddition, a surface flow length along a side surface of the column bodyis given by a mathematical expression of

[0063] S/d2 where “s” denotes a length from a stagnation point at a tipend of the column body along the side surface, while a surface localheat transfer rate is given by a mathematical expression of$\frac{Nu}{{Re}^{1/2}}$

[0064] where “Nu” denotes Nusselt number, and “Re” denotes Reynoldsnumber.

[0065]FIG. 7 shows a result of the comparison between the aforementionedcolumn bodies with respect to a relationship between the surface flowlength and surface local heat transfer rate. In addition, FIG. 8 shows aresult of comparison between the column bodies with respect to arelationship between the Reynolds number Re and a drag coefficient C_(D)representative of flow resistance. Incidentally, the column body havingthe elliptical section is referred to as an “elliptical” column body,while the column body having the circular section is referred to as a“circular” column body.

[0066] According to FIG. 7, the surface local heat transfer rate of theelliptical column body at its front-end portion (which is close to thestagnation point) has a remarkably high value as compared with thecircular column body. In addition, the surface local heat transfer rateof the elliptical column body is reduced as a flow passes by thefront-end portion to reach a back-end portion, but it is normally higherthan the surface local heat transfer rate of the circular column body.

[0067]FIG. 8 shows that the drag coefficient of the elliptical columnbody is normally lower than the drag coefficient of the circular columnbody, regardless of variations of the Reynolds number Re. Roughlyspeaking, the drag coefficient of the elliptical column body isapproximately a half of the drag coefficient of the circular columnbody.

[0068] It is preferable that the elliptical sectional shape of thecolumn 26 meets a relationship of an inequality (1), as follows:$\begin{matrix}{2.0 \leq \frac{2}{1} \leq 3.0} & (1)\end{matrix}$

[0069] where “d1” denotes a short length, and “d2” denotes a long lengthshown in FIG. 4.

[0070] In the inequality (1), as a value of d2/d1 becomes lower than2.0, the sectional shape of the column 26 is gradually changed from theelliptical shape to the circular shape, so that the surface local heattransfer rate is reduced, while the drag coefficient is increased. Incontrast, as the value of d2/d1 becomes higher than 3.0, curvature ofthe column body in proximity to its front-end portion becomes too smallto cause the foregoing flow separation, so that the surface local heattransfer rate is being reduced.

[0071] In addition, the heat exchanger 10 is designed such that thecolumns 26 are arranged inside of the refrigerant passage 23 in a zigzagmanner. Herein, refrigerant flow inside of the refrigerant passage 23 bybranches like net patterns, wherein the columns 26 are located atintersections of branches of a refrigerant flow. That is, therefrigerant flow effectively collides with front-end portions of thecolumns 26. Thus, it is possible to improve a heat transfer rate withrespect to the heat exchanger 10.

[0072] Next, comparison is made between the tube 11 (which correspondsto a tube 11A in shape, see FIG. 11A) in which a number of columns eachhaving a sectional shape meeting the aforementioned inequality (1) areformed and the conventional extrusion tube which is made by extrusionmolding with respect to heat exchange performance. Herein, two kinds ofgraphs are provided to show comparison results between them.Specifically, FIG. 9 shows a relationship between refrigerantcirculation and heat transfer rate, while FIG. 10 shows a relationshipbetween refrigerant circulation and pressure loss. Those graphs showthat both of the tube 11 having the columns and the extrusion tube aresimilarly increased in pressure loss in response to increase of therefrigerant circulation. However, it is clearly shown that as comparedwith the extrusion tube, the tube 11 is capable of remarkably increasingthe heat transfer rate in response to the increase of the refrigerantcirculation.

[0073] In FIG. 4, a reference symbol “p1” designates a center distance(or pitch) between two columns which are arranged obliquely adjacent toeach other in a direction B (corresponding to a width direction of thetube). In addition, a reference symbol “p2” designates a center distancebetween the two columns which are arranged obliquely adjacent to eachother in a direction A. According to our experimental results, thecenter distances p1, p2 should be respectively related to a short lengthd1 and a long length d2 of the column by prescribed relationships, whichare expressed by inequalities (2), (3), as follows: $\begin{matrix}{1.5 \leq \frac{p1}{d1} \leq 3.0} & (2) \\{0.5 \leq \frac{p2}{d2} \leq 1.5} & (3)\end{matrix}$

[0074] That is, it is preferable that the columns are arranged in azigzag manner to meet the aforementioned relationships.

[0075] The inequality (2) is determined by the following reasons:

[0076] If a value of p1/d1 becomes lower than 1.5, an interval ofdistance between obliquely adjacent columns in the direction B isnarrowed to increase flow resistance in the refrigerant passage 23. Ifthe value of p1/d1 becomes larger than 3.0, the interval of distancebetween the obliquely adjacent columns are broadened to decrease theflow resistance in the refrigerant passage 23, while flow speed of therefrigerant flowing between the columns is reduced to decrease the heattransfer rate.

[0077] The inequality (3) is determined by the following reasons:

[0078] If a value of p2/d2 becomes lower than 0.5, an interval ofdistance between obliquely adjacent columns in the direction A isnarrowed so that branch flows of refrigerant around the columnsinterfere with each other. This decreases the flow resistance andcorrespondingly reduces the heat transfer rate. If the value of p2/d2becomes larger than 1.5, the interval of distance between the obliquelyadjacent columns in the direction A is broadened so that branch flows ofrefrigerant at back sides of the columns are reduced. This reduces theheat transfer rate as well.

[0079] Next, comparison is made with respect to four types of tubes 11A,11B, 11C and 11D, which are different from each other in arrangement ofcolumns as shown in FIGS. 11A, 11B, 11C and 11D. Two graphs are providedto show comparison results between them. Specifically, FIG. 12 showsrelationships between refrigerant circulation and heat transfer rate,and FIG. 13 shows relationships between refrigerant circulation andpressure loss. Among the four types of the tubes, all of the columnshave a same sectional shape, in which d1=3.0 and d2=6.1.

[0080]FIG. 12 shows that substantially same values are measured withrespect to the heat transfer rate against the refrigerant circulation inthe tube A (where p1/d1≈1.5, p2/d2 ≈0.6), tube B (where p1/d1≈1.5,p2/d2≈1.15) and tube C (where p1/d1≈2.0, p2/d2≈1.15). As compared withthose tubes A, B and C, the tube D (where p1/d1≈1.27, p2/d2≈1.15) showsnormally higher values with respect to the heat transfer rate againstthe refrigerant circulation.

[0081]FIG. 13 shows that substantially same values are measured withrespect to the pressure loss against the refrigerant circulation in thetubes A, B and C. As compared with those tubes A, B and C, the tube Dshows slightly higher values with respect to the pressure loss againstthe refrigerant circulation, wherein small differences of the heattransfer rate emerge between the tube D and the other tubes (A, B, C).

[0082] In the heat exchanger 10 (see FIG. 4), all the columns 26 arearranged to be separated from each other, wherein obliquely adjacentcolumns are arranged being partly overlapped with each other in thedirection A. Such arrangement of the columns provides improvements inheat transfer rate and pressure-proof strength with respect to the tube11 as a whole. Concretely speaking, the surface local heat transfer ratemeasured along the side surface of the column is made highest at thefront-end portion and becomes lower in a direction toward the back-endportion. Consideration is made with respect to two obliquely adjacentcolumns which are obliquely arranged in the direction A, namely, anupstream column and a downstream column which are arranged at differentlocations along the refrigerant flow. Herein, the upstream column anddownstream column are arranged being partly overlapped with each otherin the direction A. That is, a front-end portion of the downstreamcolumn is located in an upstream side rather than a back-end portion ofthe upstream column. In that case, the front-end portion of thedownstream column compensates for reduction of the surface local heattransfer rate at the back-end portion of the upstream column. Thus, itis possible to improve the overall heat transfer rate of the tube 11 onthe average.

[0083] In the obliquely adjacent columns described above, the front-endportion of the downstream column is located in the upstream side ratherthan the back-end portion of the upstream column. In other words, thecolumns partly overlap with each other in arrangement in the directionA. So, any section of the tube 11 taken along a line perpendicular tothe direction A normally contain the column(s). As shown in FIG. 3, eachcolumn is made by bonding the top portions (25 a) of the swellingportions (25) respectively formed on the first and second walls 21, 22by brazing. In other words, each column acts as a joint formed betweenthe first and second walls 21, 22. Because the columns are arrangedregularly in the direction A, it is possible to secure broad jointportions between the top portions (25 a) of the swelling portions (25).For this reason, any section of the tube 11 taken in the direction Acontains adhesion between the swelling portions 25 of the first andsecond walls 21, 22. Thus, it is possible to increase joint strengthbetween the first and second walls 21, 22 of the tube 11, and it ispossible to secure a sufficiently high pressure-proof strength withrespect to the tube 11 even if the thickness of the flat plate 20 isthin.

[0084] [B] Second Embodiment

[0085] Next, a heat exchanger having a tube 11 which is designed inaccordance with a second embodiment of the invention will be describedwith reference to FIG. 13, wherein parts equivalent to those used in thefirst embodiment will be designated by the same reference numerals,hence, the description thereof will be omitted.

[0086] As shown in FIG. 14, swelling portions 42 whose sectional shapescorrespond to ellipses each having a long length and a short length areformed and arranged in a slanted manner with respect to a direction A oninterior surfaces of the tube 11. That is, each of the swelling portions42 is arranged in such a manner that the long length thereof is arrangedwith inclination to a horizontal line corresponding to the direction Aby a prescribed angle θ. As similar to the foregoing first embodiment,each pair of the swelling portions 42 are arranged to conform with eachother in elevation such that their top portions 42 are brought intocontact with each other. Thus, a column 43 is made by jointing togetherthe pair of the swelling portions 42 inside of the tube 11. In addition,the swelling portions 42 are arranged in a zigzag manner with respect tothe direction A. That is, obliquely adjacent swelling portions which arearranged obliquely adjacent to each other in the direction A arearranged independently from each other but are partly overlapped witheach other along the direction A. Thus, columns 43 are arrangedcorrespondingly in conformity with the swelling portions 42.

[0087] Like the foregoing first embodiment, the heat exchanger of thesecond embodiment is designed such that obliquely adjacent columns 43are arranged being partly overlapped with each other along the directionA in the tube 11. So, it is possible to provide improvements in heattransfer rate and pressure-proof strength of the tube 11. In addition,the second embodiment is characterized by that each of the swellingportions 42 constructing the columns 43 is arranged in a slanted mannerin which its long length is arranged with inclination to the direction Aby the angle θ. This technical feature of the second embodiment will bedescribed in detail in consideration of two columns (43), namely, anupstream column and a downstream column which are arranged adjacent toeach other but are arranged at different locations within therefrigerant flow. Herein, a front-end portion of the downstream columnis located slightly different from a back-end portion of the upstreamcolumn by a prescribed offset in a direction B (which is perpendicularto the direction A, not shown in FIG. 14). For this reason, thefront-end portion of the downstream column does not act as a “shadowzone” for the refrigerant flow. This increases an amount of refrigerantthat collide with each of front-end portions of the columns 43. Thus, itis possible to improve the heat transfer rate with respect to the tube11 as a whole.

[0088] Incidentally, it is preferable to set the inclination angle θwithin a range of ±7°. Such a range is determined by the followingreasons:

[0089] If the inclination angle is gradually increased from 0°, the heattransfer rate is correspondingly improved so that the second embodimentis able to demonstrate remarkable effects in heat-exchange property.However, when the inclination angle becomes larger or lower than therange of ±7°, flow separation is easily caused to occur in therefrigerant flow, so that the heat transfer rate is reduced.

[0090] [C] Third Embodiment

[0091] Next, a heat exchanger having a tube 11 which is designed inaccordance with a third embodiment of the invention will be describedwith reference to FIGS. 15 and 16, wherein parts equivalent to thoseused by the first embodiment are designated by the same referencenumerals, hence, the description thereof will be omitted.

[0092] Like the foregoing first embodiment, the third embodiment isbasically designed such that the tube 11 is constructed by first andsecond walls 21, 22 between which columns 26 are formed by swellingportions 25 and are arranged obliquely adjacent to each other. In FIG.15, the third embodiment is characterized by that side walls 44 areformed and arranged integrally with side-end portions of the first andsecond walls 21, 22. Therefore, a refrigerant passage 23 is formed andencompassed by those walls 21, 22, 44. In addition, semi-columns 46 eachhaving a prescribed shape corresponding to a semi-shape of theaforementioned column 26 whose sectional shape corresponds to an ellipseare arranged on the side walls 44. Each of the semi-columns 46 is formedby a pair of semi-swelling portions 45 whose top portions are broughtinto contact with each other. Herein, the semi-swelling portions 45 areformed by applying external pressures to exterior surfaces of the firstand second walls 21, 22 to partially cave in at selected positions.

[0093] Each of the semi-columns 46 whose sectional shapes correspond tosemi-ellipses is arranged in connection with the columns 26 whosesectional shapes correspond to ellipses and which are arranged in azigzag manner. That is, one semi-column 46 is arranged on the side wall44 at a prescribed location, which approximately corresponds to a centerposition between two columns (each designated by a reference numeral “26a”) being arranged adjacent to each other along a direction A within thecolumns 26. In addition, the semi-column 46 is also arranged adjacent toa column 26 b, which is arranged obliquely adjacent to the column 26 a,along a direction B.

[0094] According to the heat exchanger of the third embodiment havingthe tube 11 in which the semi-columns 46 each having the semi-shape ofthe column 26 are arranged on the side walls 44, it is possible toprovide improvements in heat transfer rate and pressure-proof strengthof the tube 11. Concretely speaking, the columns 26 whose sectionalshapes correspond to ellipses are arranged in a zigzag manner along thedirection A in the tube 11, wherein one or two columns 26 are arrangedin each section taken along the direction B. In other words, there aretwo kinds of sections each taken along the direction B, namely, a firstsection in which two columns 26 a are arranged and a second section inwhich one column 26 b is arranged. Those sections are arrangedalternately along the direction A in the tube 11. As compared with thefirst section having the two columns 26 a, the second section having thecolumn 26 b is reduced in joint strength because of a small total jointarea formed between the first and second walls 21, 22 which are jointedtogether by the column 26 b. In other words, the second section havingthe column 26 b is reduced in pressure-proof strength as compared withthe first section having the two columns 26 a. To compensate reductionof the pressure-proof strength, the semi-columns 46 each having asemi-shape of the column 26 are arranged in connection with the secondsection having the column 26 b so as to increase a total joint areabetween the first and second walls 21, 21 which are jointed together bythe column 26 b and two semi-columns 46 with respect to the secondsection. Therefore, it is possible to increase the joint strength withrespect to the second section. In other words, it is possible toincrease the pressure-proof strength of the second section beingsubstantially equivalent to the pressure-proof strength of the firstsection having the two columns 26 a.

[0095] By provision of the semi-columns 46, turbulence is caused tooccur in refrigerant flows along the side walls 44, so it is possible toimprove an overall heat transfer rate of the tube 11 because ofincreasing turbulence effects.

[0096]FIG. 16 shows a modified example of the heat exchanger of thethird embodiment, which is designed as a laminated heat exchanger usedfor an evaporator. Herein, the heat exchange of FIG. 16 has arefrigerant passage unit 47 equipped with a U-shaped refrigerant passage50 having a refrigerant inlet 48 and a refrigerant outlet 49 at upperends. That is, refrigerant is introduced into the refrigerant inlet 48to flow inside of the U-shaped refrigerant passage 50, wherein itfirstly flows down to a lower end and then flows upwardly toward therefrigerant outlet 49. The U-shaped refrigerant passage 50 is not formedin a straight shape like the foregoing refrigerant passage 23 but isbasically designed to have columns as similar to the refrigerant passage23 inside of the tube 11 shown in FIG. 15. That is, semi-columns arearranged along side walls of the refrigerant passage 50. Thus, it ispossible to improve pressure-proof strength and heat transfer rate withrespect to the refrigerant passage unit 47.

[0097] [D] Fourth Embodiment

[0098] Next, a heat exchanger having a tube 11 which is designed inaccordance with a fourth embodiment of the invention will be describedwith reference to FIG. 17, wherein parts equivalent to those used by thefirst embodiment are designated by the same reference numerals, hence,the description thereof will be omitted.

[0099] The heat exchanger of the fourth embodiment is designed as acondenser that condenses refrigerant by radiating heat to the externalair. The present heat exchanger uses the tube 11 shown in FIG. 17, whichis characterized by that each of swelling portions 25 is graduallyenlarged in size along a direction A while maintaining figure similarityin sectional shape. Along with the direction A, relatively smallswelling portions are formed and arranged in an upstream side, whilerelatively large swelling portions are formed and arranged in adownstream side. Hence, densities (or occupied areas) of the swellingportions in the upsteam side are relatively small, while the swellingportions are closely and tightly arranged with each other in thedownstream side. Therefore, columns 26 are correspondingly formed andarranged in coformity with the swelling portions 25. As a result,sectional areas of a refrigerant passage 23 taken along linesperpendicular to the direction A become small in the direction A fromthe upstream side to the downstream side of the tube 11.

[0100] In the case of the heat exchanger that is designed as thecondenser, dryness is reduced in response to progress of refrigerantthat flow from the upstream side to the downstream side, in other words,a liquid phase is increased as compared with a gas phase in response tothe progress of the refrigerant. For this reason, pressures which areimparted to interior wall surfaces of the tube 11 by refrigerant aregradually reduced along the direction A. To compensate reduction of thepressures, the tube 11 used by the heat exchanger of the fourthembodiment is designed such that sectional areas of the refrigerantpassage 23 are gradually reduced in response to the reduction of thepressures. So, it is possible to provide substantially constantpressures being imparted to the interior wall surfaces of the tube 11.Thus, it is possible to secure substantially a constant heat transferrate having a relatively high value within an overall area of the tube11 in its length direction. In addition, it is possible to reducepressure loss being constantly low within the overall area of the tube11 in its length direction.

[0101] As described above, the tube 11 of the fourth embodiment ischaracterized by that the columns 26 are made being gradually enlargedin sizes while maintaining a certain figure similarity in the directionA directing from the upstream side to the downstream side. So, thesectional areas of the refrigerant passage 23 taken along linesperpendicular to the direction A are made being gradually reduced in thedirection A from the upstream side to the downstream side. The fourthembodiment can be modified such that the columns 26 are changed in sizeas well as shape without maintaining figure similarity. Or, it can bemodified such that the columns 26 are not changed in sizes but arechanged in arrangement (or density) in the direction A.

[0102] [E] Fifth Embodiment

[0103] Next, a heat exchanger 10 which is designed in accordance with afifth embodiment of the invention will be described with reference toFIG. 18.

[0104] The heat exchanger of the fifth embodiment is designed as anevaporator that absorbs heat from the external air to gasifyrefrigerant. The present heat exchanger is constructed by laminatingrefrigerant passage units 53, each of which is formed by overlappingtogether flat plates 51, 52 each roughly having a rectangular shape asshown in FIG. 18. Herein, the flat plates 51, 52 are assembled togetherby jointing their peripheral portions and center portions together.Thus, a U-shaped refrigerant passage 56 which is shaped like a flat tubeis formed in the refrigerant passage unit 53 having a refrigerant inlet54 and a refrigerant outlet 55 at upper ends. Thus, refrigerant isintroduced into the refrigerant inlet 54 to flow inside of the U-shapedrefrigerant passage 56, wherein it flows down to a lower end and thenflows upwardly toward to the refrigerant outlet 55.

[0105] When the center portions of the flat plates 51, 52 are jointedtogether, a partition portion 57 is formed to partition the refrigerantpassage 56 into two sections (i.e., a right section and a left sectionin FIG. 18). Herein, the partition portion 57 is formed in a slantedmanner. That is, a lower end 57 b of the partition portion 57 isarranged substantially at a center with an equal distance being measuredfrom both ends of the flat plates 51, 52, while an upper end 57 a of thepartition portion 57 is arranged close to the refrigerant inlet 54rather than the refrigerant outlet 55. As a result, sectional areas ofthe refrigerant passage 56 taken along lines perpendicular to a flowdirection of refrigerant are made small in upstream areas but are madelarge in downstream areas. That is, the sectional shapes of therefrigerant passage 56 are gradually increased along refrigerant flowfrom an upstream side to a downstream side.

[0106] In addition, external wall surfaces of the flat plates 51, 52which are arranged opposite to each other are pressed to cave in atselected positions to form a number of swelling portions 58. Therefore,plural columns 59 are formed by jointing together top portions of thecorresponding swelling portions 58, which are formed on interior wallsurfaces of the flat plates 51, 52 and are arranged in connection witheach other.

[0107] In the refrigerant passage 56, the columns 59 are uniformlyarranged to maintain constant distances in a refrigerant flow directionand its perpendicular direction. That is, a constant distance ismaintained between adjacent columns 59 in the refrigerant flowdirection. In addition, a constant distance is also maintained betweenadjacent columns 59 in a direction perpendicular to the refrigerant flowdirection. Due to such uniform arrangement of the columns 59 and aslanted arrangement of the partition portion 57, it is possible to makesectional areas of the refrigerant passage 56, taken along linesperpendicular to the refrigerant flow direction, being larger in adirection from the upstream side to the downstream side.

[0108] In the case of the heat exchanger which is designed as theevaporator, dryness is increased in response to progress of refrigerantthat flow from the upstream side to the downstream side, in other words,gas phase is increased as compared with liquid phase in response to theprogress of the refrigerant. For this reason, pressures imparted tointerior wall surfaces of the refrigerant passage 56 are graduallyincreased in the refrigerant passage unit 53. To cope with increases ofthe pressures, the heat exchanger of the fifth embodiment using therefrigerant passage unit 53 is designed such that the sectional areas ofthe refrigerant passage 56 are made gradually larger in response to theincreases of the pressures. Thus, it is possible to secure substantiallya constant heat transfer rate having a relatively high value within anoverall area of the refrigerant passage 56 in its refrigerant flowdirection. In addition, it is possible to reduce pressure loss beingconstantly low within the overall area of the refrigerant passage 56 inits refrigerant flow direction.

[0109] In the aforementioned refrigerant passage unit 53, the columns 59are uniformly arranged in the refrigerant passage 56 such that aconstant distance is maintained between the adjacent columns, so thatthe sectional areas of the refrigerant passage 56 are graduallyincreased in the refrigerant flow direction from the upstream side tothe downstream side. The fifth embodiment can be modified such that thecolumns 59 are subjected to uniform arrangement but are graduallyenlarged in size along the refrigerant flow direction toward thedownstream side. Or, it can be modified such that the columns 59 are notchanged in size but are gradually increased in number along therefrigerant flow direction toward the downstream side, in other words,densities of the columns 59 are gradually increased along therefrigerant flow direction toward the downstream side.

[0110] As described heretofore, this invention has a variety oftechnical features and effects, which are summarized as follows:

[0111] (1) A heat exchanger of this invention basically uses tubes, eachof which is designed such that a number of columns are arranged insideof a refrigerant passage and are made by jointing together top portionsof swelling portions of first and second walls, which are arrangedopposite to each other. According to one aspect of the invention,adjacent columns are arranged at different locations in a refrigerantflow in such a way that a front-end portion of a downstream column isarranged in an upstream side as compared with a back-end portion of anupstream column. Herein, the front-end portion of the downstream columncompensates for reduction of a surface local heat transfer rate at theback-end portion of the upstream column. Thus, it is possible to improvean overall heat transfer rate of the tube on the average.

[0112] (2) Because the adjacent columns are arranged such that thefront-end portion of the downstream column is arranged in the upstreamside as compared with the back-end portion of the upstream column, thecolumns normally exist being partly overlapped with each other in anysections of the tube being taken along lines perpendicular to its lengthdirection, in other words, the swelling portions of the first and secondwalls are bonded together at any sections of the tube. Thus, it ispossible to improve a joint strength for jointing the first and secondwalls together as well as a pressure-proof strength of the tube as awhole.

[0113] (3) According to a second aspect of the invention, semi-columnsare arranged on side walls of the tube constructed by the first andsecond walls and are made by jointing together top portions ofsemi-swelling portions. This increases joint areas between the first andsecond walls, so it is possible to increase an overall joint strengthbetween the first and second walls. By provision of the semi-columns onthe side walls of the tube, turbulence is caused to occur in refrigerantflows along the side walls. This increases turbulent effects, so it ispossible to improve an overall heat transfer rate with respect to thetube.

[0114] (4) According to a third aspect of the invention, the columnseach having an elliptical sectional shape having a long length and ashort length are formed and arranged in a slanted manner such that thelong length is slanted with a certain angle of inclination to the lengthdirection of the tube. This provides an offset in a width direction ofthe tube between the front-end portion of the downstream column and theback-end portion of the upstream column. In other words, the front-endportion of the downstream column does not act as a shadow zone in therefrigerant flow. That is, it is possible to increase amounts ofrefrigerant colliding with front-end portions of the columns, so it ispossible to improve an overall heat transfer rate with respect to thetube.

[0115] (5) In order to use the heat exchanger as the condenser, thecolumns arranged inside of the tube are gradually increased in number ordensity along the refrigerant flow direction, so that sectional areas ofthe refrigerant passage taken along lines perpendicular to a lengthdirection of the tube are gradually reduced in response to pressures,which are imparted to interior wall surfaces of the tube and which aregradually reduced in a refrigerant flow direction from an upstream sideto a downstream side. Therefore, it is possible to stabilize thepressures being substantially constant. Thus, it is possible to securesubstantially a constant heat transfer rate having a relatively highvalue within an overall area of the tube in its length direction. Inaddition, it is possible to reduce pressure loss being constantly lowwithin the overall area of the tube in its length direction.

[0116] (6) In order to use the heat exchanger as the evaporator, thecolumns arranged inside of the tube are gradually decreased in number ordensity in the refrigerant flow direction, so that the sectional areasof the refrigerant passage are gradually enlarged in response topressures, which are imparted to the interior wall surfaces of the tubeand which are gradually increased in the refrigerant flow direction fromthe upstream side to the downstream side. Therefore, it is possible tostabilize the pressures being substantially constant. Thus, it ispossible to secure substantially a constant heat transfer rate having arelatively high value within an overall area of the tube in its lengthdirection. In addition, it is possible to reduce pressure loss beingconstantly low within the overall area of the tube in its lengthdirection.

[0117] As this invention may be embodied in several forms withoutdeparting from the spirit of essential characteristics thereof, thepresent embodiments are therefore illustrative and not restrictive,since the scope of the invention is defined by the appended claimsrather than by the description preceding them, and all changes that fallwithin metes and bounds of the claims, or equivalence of such metes andbounds are therefore intended to be embraced by the claims.

What is claimed is:
 1. A heat exchanger comprising: a flat tubeconstructed by a first wall and a second wall which are arrangedopposite and apart in parallel with each other and are assembledtogether to form a refrigerant passage; and a plurality of columnsformed inside of the flat tube, wherein each of the plurality of columnsis formed by joining together top portions of swelling portions, whichswell from interior surfaces of the first and second walls by applyingexternal pressures to exterior surfaces of the first and second walls tocave in respectively and which are arranged opposite in connection witheach other inside of the flat tube, and wherein each of the plurality ofcolumns has a prescribed sectional shape which corresponds to anelliptical shape or an elongated circular shape each defined by a shortlength and a long length, wherein the plurality of columns are arrangedto align long lengths thereof along a length direction of the flat tubein such a manner that obliquely adjacent columns, which are arrangedadjacent to each other obliquely with respect to the length direction ofthe tube, are arranged at different locations but are partly overlappedwith each other with long lengths thereof in view of a width directionperpendicular to the length direction of the flat tube.
 2. A heatexchanger according to claim 1 wherein the flat tube is constructedusing side walls which are arranged at side ends of the first and secondwalls and on which a plurality of semi-columns each having a semi-shapeof the column are formed in connection with the plurality of columns insuch a manner that each of the plurality of semi-columns is arrangedobliquely adjacent to a selected column to partly overlap with itsfront-end portion or its back-end portion.
 3. A heat exchangercomprising: a flat tube constructed by a first wall and a second wallwhich are arranged opposite and apart in parallel with each other andare assembled together to form a refrigerant passage; and a plurality ofcolumns each having a prescribed sectional shape corresponding to anelliptical shape or an elongated circular shape each defined by a shortlength d1 and a long length d2, wherein the plurality of columns arearranged between the first and second walls and are arranged to alignlong lengths thereof along a length direction of the flat tube such thatobliquely adjacent columns, which are arranged adjacent to each otherobliquely with respect to the length direction of the flat tube, arearranged at different locations but are partly overlapped with eachother with long lengths thereof in view of a width directionperpendicular to the length direction of the flat tube, wherein each ofthe plurality of columns has the prescribed sectional shape which isdefined by a relationship of ${2.0 \leq \frac{2}{1} \leq 3.0},$

and wherein using a first center distance p1 being measured between theobliquely adjacent columns in the width direction of the flat tube and asecond center distance p2 being measured between the obliquely adjacentcolumns in the length direction of the flat tube, the plurality ofcolumns are arranged to meet relationships of$1.5 \leq \frac{p1}{d1} \leq {3.0\quad {and}\quad 0.5} \leq \frac{p2}{d2} \leq {1.5.}$


4. A heat exchanger comprising: a flat tube constructed by a first walland a second wall which are arranged opposite and apart in parallel witheach other and are assembled together to form a refrigerant passage; aplurality of columns each having a prescribed sectional shapecorresponding to an elliptical shape or an elongated circular shape eachdefined by a short length and a long length, wherein the plurality ofcolumns are arranged between the first and second walls and are arrangedto align long lengths thereof being slanted with respect to a lengthdirection of the flat tube such that the long length of the column isslanted with a prescribed angle of inclination to the length directionof the flat tube.
 5. A heat exchanger according to claim 4 wherein theprescribed angle of inclination is set within a range of ±7°.
 6. A heatexchanger comprising: a flat tube constructed by a first wall and asecond wall which are arranged opposite and apart in parallel with eachother and are assembled together to form a refrigerant passage; and aplurality of columns each having a prescribed sectional shapecorresponding to an elliptical shape or an elongated circular shape eachdefined by a short length d1 and a long length d2, wherein each of theplurality of columns is formed by joining together top portions ofswelling portions which swell from interior surfaces of the first andsecond walls by applying external pressures to exterior surface of thefirst and second walls respectively and which are arranged opposite inconnection with each other inside of the flat tube, and wherein theplurality of columns are arranged to align long lengths thereof along alength direction of the flat tube such that obliquely adjacent columns,which are arranged adjacent to each other obliquely with respect to thelength direction of the flat tube, are arranged at different locationsbut are partly overlapped with each other with long lengths thereof inview of a width direction perpendicular to the length direction of theflat tube, wherein each of the plurality of columns has the sectionalshape which is defined by a relationship of${2.0 \leq \frac{2}{1} \leq 3.0},$

and wherein using a first center distance p1 being measured between theobliquely adjacent columns in the width direction of the flat tube and asecond center distance p2 being measured between the obliquely adjacentcolumns in the length direction of the flat tube, the plurality ofcolumns are arranged to meet relationships of$1.5 \leq \frac{p1}{d1} \leq {3.0\quad {and}\quad 0.5} \leq \frac{p2}{d2} \leq {1.5.}$


7. A heat exchanger comprising: a flat tube constructed by a first walland a second wall which are arranged opposite and apart in parallel witheach other and are assembled together to form a refrigerant passage; anda plurality of columns each having a prescribed sectional shapecorresponding to an elliptical shape or an elongated circular shape eachdefined by a short length and a long length, wherein each of theplurality of columns is formed by joining together top portions ofswelling portions which swell from interior surfaces of the first andsecond walls by applying external pressures to exterior surface of thefirst and second walls respectively and which are arranged opposite inconnection with each other inside of the flat tube, wherein theplurality of columns are arranged to align long lengths thereof beingslanted with respect to a length direction of the flat tube such thatthe long length of the column is slanted with a prescribed angle ofinclination to the length direction of the flat tube, and wherein theplurality of columns are arranged to adjoin with each other such thatobliquely adjacent columns, which are arranged adjacent to each otherobliquely with respect to the length direction of the flat tube, arearranged at different locations but are partly overlapped with eachother with long lengths thereof in view of a width directionperpendicular to the length direction of the flat tube.
 8. A heatexchanger according to claim 7 wherein the prescribed angle ofinclination is set within a range of ±7°.
 9. A heat exchangercomprising: a flat tube constructed by a first wall and a second wallwhich are arranged opposite and apart in parallel with each other andare assembled together to form a refrigerant passage; and a plurality ofcolumns each having a prescribed sectional shape corresponding to anelliptical shape or an elongated circular shape each defined by a shortlength and a long length, wherein the plurality of columns are arrangedbetween the first and second walls and are arranged to align longlengths thereof in a length direction of the flat tube and wherein theplurality of columns are arranged in a gradually concentrated manneralong a refrigerant flow direction corresponding to the length directionof the flat tube.
 10. A heat exchanger according to claim 9 wherein theplurality of columns are arranged to gradually increase in number, sizeor density in the refrigerant flow direction.
 11. A heat exchangercomprising: a flat tube constructed by a first wall and a second wallwhich are arranged opposite and apart in parallel with each other andare assembled together to form a refrigerant passage; and a plurality ofcolumns each having a prescribed sectional shape corresponding to anelliptical shape or an elongated circular shape each defined by a shortlength and a long length, wherein the plurality of columns are arrangedbetween the first and second walls and are arranged to align longlengths thereof in a length direction of the flat tube and wherein theplurality of columns are arranged in a gradually deconcentrated manneralong a refrigerant flow direction corresponding to the length directionof the flat tube.
 12. A heat exchanger according to claim 11 wherein theplurality of columns are arranged to gradually decrease in number, sizeor density in the refrigerant flow direction.
 13. A heat exchangercomprising: a plurality of tubes each of which has a flat shape andcontains a plurality of columns therein, wherein each of the pluralityof columns has a prescribed sectional shape corresponding to anelliptical shape or an elongated circular shape each defined by a shortlength and a long length and wherein the plurality of columns arearranged to align long lengths thereof along a length direction of thetube such that obliquely adjacent columns, which are arranged adjacentto each other obliquely with respect to the length direction of thetube, are arranged at different locations but are partly overlapped witheach other with long lengths thereof in view of a width directionperpendicular to the length direction of the tube; a plurality ofcorrugated fins, each of which is arranged between the tubes such thatcrest portions are brought into contact with an exterior surface of thetube; and two head pipes which are arranged apart from each other with aprescribed interval of distance therebetween and between which theplurality of tubes and the plurality of corrugated fins are arranged, sothat both ends of a refrigerant passage formed inside of the tubecommunicate with insides of the two head pipes respectively.
 14. A heatexchanger according to claim 13 wherein each of the plurality of tubesis constructed by bending a flat plate to form a first wall and a secondwall, which are arranged opposite to each other with a prescribedinterval of distance therebetween to provide the refrigerant passage,wherein swelling portions are formed to swell from interior surfaces ofthe first and second walls by applying external pressures to the firstand second walls respectively and are arranged at selected positions bywhich top portions of the swelling portions are correspondingly broughtinto contact with each other to form the columns.
 15. A heat exchangeraccording to claim 14 wherein the first and second walls are bondedtogether at selected portions by brazing to form the tube, while the topportions of the swelling portions are bonded together by brazing to formthe columns.
 16. A manufacturing method of a heat exchanger comprisingthe steps of: constructing a plurality of tubes each having a flat shapeby bending flat plates whose surfaces are clad with brazing material,wherein a plurality of swelling portions are formed to swell frominterior surfaces of the tube and their top portions are correspondinglypaired and brought into contact with each other to form a plurality ofcolumns inside of the tube; providing a plurality of corrugated fins,which are respectively arranged between the plurality of tubes;assembling the plurality of tubes and the plurality of corrugated finstogether with two head pipes such that the plurality of tubes and theplurality of corrugated fins are alternatively arranged in elevation andare horizontally held between the two head pipes, wherein each of theplurality of tubes having refrigerant passages communicates with the twohead pipes respectively; and placing the plurality of tubes, theplurality of corrugated fins and the two head pipes which are assembledtogether into a heating furnace to heat for a prescribed time.
 17. Amanufacturing method of the heat exchanger according to claim 16 furthercomprising the steps of: forming the plurality of swelling portions toswell from an interior surface of the flat plate at selected positionsby press; and bending the flat plate to form a first wall and a secondwall, which are arranged opposite to each other with a prescribedinterval of distance therebetween to form a tube, wherein the first andsecond walls are adjusted in position such that the plurality ofswelling portions are correspondingly paired with each other inelevation and top portions thereof are correspondingly brought intocontact with each other to form a plurality of columns inside of thetube.
 18. A manufacturing method of the heat exchanger according toclaim 16 wherein the plurality of columns each having a prescribedsectional shape corresponding to an elliptical shape or an elongatedcircular shape defined by a short length d1 and a long length d2 arearranged to align long lengths thereof in a length direction of the tubesuch that obliquely adjacent columns, which are arranged adjacent toeach other obliquely with respect to the length direction of the tube,are arranged at different locations but are partly overlapped with eachother with long lengths thereof in view of a width directionperpendicular to the length direction of the tube, wherein each of theplurality of columns is defined in sectional shape by a relationship of$2.0 \leq \frac{2}{1} \leq {3.0.}$

and wherein using a first center distance p1 being measured between theobliquely adjacent columns in the width direction of the tube and asecond center distance p2 being measured between the obliquely adjacentcolumns in the length direction of the tube, the plurality of columnsare arranged to meet relationships of$1.5 \leq \frac{p1}{d1} \leq {3.0\quad {and}\quad 0.5} \leq \frac{p2}{d2} \leq {1.5.}$